1. Field of the Invention
The present invention relates to a method for extracting automatically pertinent information for sedimentologic interpretation, in order to reconstruct the depositional conditions under which sedimentary layers have formed.
2. Description of the Prior Art
Sedimentology reconstructs environments wherein the various rocks samples of which are studied have formed. The subsoil has of piles of sedimentary layers that have settled in sub-horizontal position. The nature of the rocks that make up these layers can vary from one layer to the next, notably depending on the water depth under which the sediment has settled. These layers have a limited lateral extension because the sedimentation process does not affect the whole of the earth's surface at a given time of the geologic history.
Stratigraphy is that branch of geology that studies the order in which the rock layers that make up the earth's crust have formed through the geologic times, in particular the sedimentary rocks. It allows establishing a relative chronology referred to as stratigraphic chronology, notably through the reasoned use of two principles: a layer has the same age throughout its length and breadth, and the lower of two superposed layers is also the older. Thus units are obtained that can be differentiated according to the nature of the arguments used to define them: lithostratigraphic units, biostratigraphic units and chronostratigraphic units. The latter are characterized by sets of layers that are made to agree with time intervals (referred to as geochronological units).
On the other hand, seismic imagery is a method of observing the architecture of these layers. This technique is based on the emission of acoustic signals in the subsoil and recording of the signals propagated and reflected on particular acoustic reflectors. These signals are processed so as to form a two or three-dimensional seismic image. This seismic image is a series of vertical records referred to as seismic traces. These seismic traces represent the amplitude of the signal received as a function of time. The recorded signals generally correlate from one trace to the next, which is translated, in a seismic image, into sub-horizontal lines, more or less rectilinear, thick and continuous, referred to as lineations. These lines represent the interfaces between sedimentary layers. Thus, a two-dimensional seismic image corresponds to a vertical section of the subsoil. An example of a two-dimensional seismic image is given by FIG. 1.
The sedimentary layers can be worn away by erosion and deformed by the tectonic history of the subsoil after deposition. Thus, the current architecture of the layers shown by seismic imagery can reach a high degree of complexity which makes the task very complicated when trying to reconstruct the geologic history undergone thereby.
Now, in the field of oil exploration, one of the objectives of the interpretation of these images (referred to as seismic stratigraphy) is precisely to analyze the architecture observed to reconstruct the history of the sedimentary deposits, in order to locate the sediments that are likely to constitute oil reservoirs. The criteria used to identify hydrocarbon-containing layers are essentially based on the estimation of the position of the sea level at the time of their deposition. For example, the sandiest sediments are generally located at the deltas found along the coastlines of the continents. These coastlines vary in the course of geologic time as a function of the sea level variations and of tectonic movements such as subsidence (earth's crust collapse) or the uplift of mountain ranges.
From the observation of seismic images, the interpreter has a certain number of more or less empirical rules for determining the water depth below which a sediment has settled. This judgment is notably based on the analysis of the relative position of the layers or on the way these layers are organized: parallel layers can indicate a regular deposit at great depth, secant layers can indicate an emersion stage interposed between the depositional stages, etc.
Interpretation is often very complicated and it entirely depends on the interpreter's sedimentologic expertise. There are few tools providing assistance in this task, which is all the more complex as a seismic image often shows several hundred layers.
The base principles of seismic stratigraphy, that is the sedimentologic interpretation of seismic images, were set out by Vail et al. in the 70s (Vail P. R., et al., 1977, “Seismic Stratigraphy and Global Changes of Sea Level”, in C. E. Payton ed., “Seismic Stratigrapy Application to Hydrocarbon Exploration”: American Association of Petroleum Geologists Memoir 26, p. 49–212). The concepts proposed by Vail et al. have not changed much since, but seismic acquisition surveys, notably for acquisition of seismic data in three dimensions, have spread significantly in the field of hydrocarbon exploration and production. Their large number requires a decrease in the time spent by the interpreter for processing these data.
Interpretation of the acquired data is performed by an interpreter trained in the principles of seismic stratigraphy. Considering the large amount of data to be processed, interpretation is carried out by means of softwares referred to as “seismic interpretation stations”. These seismic interpretation stations provide three functionality categories concerning the invention:
(1) Tools allowing semi-automatic extraction of seismic reflectors from a “seed” picked by the interpreter on the analyzed image. Research work has been undertaken for several years now for global and entirely automated extraction of all the interfaces. Examples thereof are the method described in French Patent 2,646,520 and corresponding U.S. Pat. No. 5,148,494 or more recent work described in:
M. Faraklioti, M. Petrou, “Horizon Picking in 3DSeismic Data Volumes”, Machine Vision and Applications, vol. 15, No. 4, October 2004.
(2) Tools for calculating seismic attributes (scalar or vector quantities estimated by mathematical processing). Examples thereof are the public report “TriTex IST-1999-20500, Feasability and Literature Study”, November 2001, which assesses the calculation of attributes referred to as “texture attributes”, or the review “The Leading Edge”, October 2002, vol. 21, No. 10, which presents a series of articles on seismic attributes.
(3) Chronostratigraphic interpretation methods wherein the chronology of deposition of the sedimentary deposits is estimated. An example thereof is the method described by N. Keskes in French Patent 2,808,336 and corresponding U.S. Pat. No. 6,771,800. This method is based on the calculation of a vector field applied onto the image and which describes at any point the mean local orientation of the reflectors in the vicinity of this point. A network of lines tangential at any point to this vector field, the “flowlines”, is then calculated. The local density of the flowlines is then interpreted in terms of rate of sedimentation. It can be noted that this method cannot be implemented if the initial image corresponds to a geologic medium comprising stratigraphic discontinuities such as faults, because no flowline can be propagated through these discontinuities. It can also be noted that this method provides only one chronostratigraphic interpretation out of all the possible interpretations; notably, it does not allow providing of a chronostratigraphic interpretation respecting a priori flowlines imposed by the operator.